Method of non-destructive examination of specimens

ABSTRACT

THE RESONANT FREQUENCY AND THE DAMPING FACTOR OF A SPICEMEN HAVE BEEN USED TO EXAMINE THE CONTENTS AND STRUCTURAL CONDITION OF A SPECIMEN, BY A METHOD OF DETECTING AND COUNTING THE NUMBER OF OSCILLATIONS OCCURING DURING SELECTED TIME INTERVALS DURING THE ATTENUATION OF THE OSCILLATIONS.

Nov. 30, 1971 KOICHI SUGIMOTO 3,623,358

METHOD OF NON-DESTRUCTIVE EXAMINATION OF SPECIMENS Filed Feb. 20, 1970 3 SheetsSheet 1 N i AaEXP Mi -N INVENTOR KOICHI SUGIMOTO ATTORNEY Nov. 30, 1971 KOICHI SUGIMOTO 3,623,353

METHOD OF NON'DESTRUCTIVE EXAMINATION OF SPECIMENS Filed Feb. 20, 1970 v 3 Sheets-Sheet 2 GATED COUNTER 7 t INVENTOR KOICHI SUGIMOTO ATTORNEY Nov. 30, 1971 KOICHI SUGIMOTO 3,

METHOD OF NON-DESTRUCTIVE EXAMINATION OF SPECIMENS 3 Sheets-Sheet 5 Filed Feb. 20, 1970 INVENTOR KOICHI SUGIMOTO ATTORNEY United States Patent 3,623,358 METHOD OF NON-DESTRUCTIVE EXAMINATION 0F SPECIMENS Koichi Sugimoto, Osaka, Japan, assignor to Iwatani & Co., Ltd., Osaka, Japan Filed Feb. 20, 1970, Ser. No. 13,038 Int. Cl. Gtlln 29/00 US. Cl. 7367.2 5 Claims ABSTRACT OF THE DISCLOSURE The resonant frequency and the damping factor of a specimen have been used to examine the contents and structural condition of a specimen, by a method of detecting and counting the number of oscillations occurring during selected time intervals during the attenuation of the oscillations.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method of non-destructive examination of a specimen, and particularly to a method of determining the contents of the specimen material and examining the structural condition of the specimen by measuring the characteristic resonant frequency and the damping factor after striking the specimen.

Description of the prior art In general, a specimen rings when struck on its surface. The resulting sound wave has a characteristic or natural frequency for the specimen, and diminishes progressively in amplitude with time, the rate of diminishing with time being also characteristic of the specimen. The characteristic resonant frequency and the damping factor 'vary observably with variations in the material and structural condition of the specimens. This principle is known, and has been proposed for use in such nondestructive examination. For example, a specimen has been tapped on the surface, and examined acoustically by ear to determine the contents and structural condition. For instance, this method has been applied to examine the degree of bond between a bearing metal and its linings, or for the existence of blowholes in the castings. However, this method is only usea'ble by those who have been specially trained to hear such effects, and the resulting data is not always reliable by reason of variability.

There is a further proposal, in which the resonant frequency and the damping factor of the sound are exactly measured with regularity by using an electric device. For example, a variable-frequency vibrator is used to vibrate a specimen with a low-frequency oscillator, in which the relationship between the oscillating frequency f and the amplitude A of the specimen is expressed in a resonance curve as depicted in FIG. 1, and the damping capacity Q is given by the formula:

where f is the resonance frequency, given by the peak point on the curve, and the oscillating frequencies f and f are given by the amplitudes A and A of half-power points positioned symmetrically about the peak point.

However, the mechanism and operation becomes complicated by the provision of an electric device, such as a low-frequency oscillator. As it is arranged for the specimen to be suspended on the top of a needle-like element, a heavy specimen is unfit for this method, and specimens are greatly limited in weight, shape and dimension. A further disadvantage is that each resonance curve must be depicted for each resonant frequency and damping 3,623,358 Patented Nov. 30, 1971 factor to be sought, in order that the respective values can be determined. But when the damping factor is extremely low, the difficulty becomes great because of the frequency characteristic in which the peak point is uncertain with respect to location as shown in FIG. 2. This uncertainty occurs because a too-abrupt curve in the neighborhood of the resonant frequency results in the impossibility of determining the resonance frequency f and the damping capacity Q which otherwise requires a high degree of stability of the oscillating frequency, thus rendering the unit itself very expensive.

SUMMARY OF THE INVENTION One object of the invention is to provide methods of non-destructive examination of the material and the structural condition of specimens with ease and regularity.

According to the present invention, such methods are provided, comprising striking a specimen on the surface, the specimen being supported in such a manner as to be capable of free-decay vibrations, converting the vibra tions produced from the strike into electrical signals, counting the number of oscillations occurring during the selected time intervals during the attenuation of the vibration, thereby measuring the damping factor of the specimen, and measuring the frequency of the detected signals, thereby measuring the resonant frequency of the specimen.

More particularly, when a specimen is hit on the surface, causing a free-decay vibration, the relation between the time of diminishing from A to A (where A is less than A and the amplitude A is given by:

f resonant frequency exp: natural logarithm 1r: circular constant (pi=3.l4) Q damping factor When the oscillation diminishes in amplitude, assume the value of amplitude A to be: A exp (1r). Then, the relation is established as follows:

t 1 f 0 Q When the number of oscillations N f r.

Thus, the relationship N :Q is given.

Reversely reasoned, it is seen that the final equation can lead to determining the damping factor Q from the number of oscillations N, which corresponds to the time 2 required for the Wave to diminish from A to A, which satisfactorily meets the equation; A=A exp (1r). In other words, the damping factor Q is easily measured by counting the number of the oscillations N required for the time t to diminish from A to A exp (1r). Also, the resonant frequency f is easily obtained from f N/t.

BRIEF DESCRIPTION OF THE DRAWINGS The method according to the invention will now be particularly described by way of example, with reference to the accompanying drawings.

FIGS. 1 and 2 are graphs of the frequency characteristic obtained in accordance with the prior-art method.

FIG. 3 is a graph of a waveform illustrating the principle of the present invention.

FIG. 4 is a schematic diagram of an example of an embodiment of the invention.

FIG. 5 is a schematic view of the specimen resting on a support indicating the location to be struck.

FIG. 6 is a waveform graph of level displacement in a selective amplifier.

FIG. 7 is a waveform of level displacement in a relay.

FIG. 8 is a waveform of level displacement in a rectangular wave circuit.

FIG. 9 is a waveform of level displacement in a comparison circuit.

FIG. 10 is a waveform graph of level displacement in a rectangular wave circuit.

FIG. 11 is a waveform graph of the input signals to a counter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 4, a specimen is placed on two filaments 2 and 2' in such a manner as to cause a freedecay vibration when struck, the filaments being supported with tension on opposite sides of a box 1. Alternatively, the specimen can be suspended on a filament, or it can be placed on an appropriate number of pointed bases. It is required, however, that the specimen be supported at its characteristic nodes. For example, when the specimen is a bar or plate, the characteristic nodes are located at the distance of 0.224 L (where L is length) from its respective terminating end, at which points the specimen is to be supported when a movement as shown in FIG. 5 is desired. A stroke is applied between the nodes to the central part, which is termed the loop of vibration. In FIG. 4, the arrow indicates the spot at which a stroke is directed. An electromagnetic pick-up 4 is supported extended at right angle to the specimen 3, by means of a holder 5'. The electromagnetic pick-up converts magnetic variation into electric signals. A specimen of metal or non-metal can be examined, but if a specimen, such as a wooden plate, is not SLlfilClCIltlY magnetizable, a material capable of strong magnetism, such as a piece of iron, must be attached adjacent to the electromagnetic pick-up 4. The electric output signals from the electromagnetic pick-up 4 are transmitted to a selective amplifier 7 in the amplification section 6, where the required frequency band is selected, and the amplified signals are transmitted to the operation section 8. In the operation section 8, the amplified signals from the selective amplifier 7 are received by a voltage divider 9 whose resistance is variable within a certain range. In this divider, the voltage is reduced to a predetermined value E causing a relay 10 to open switch A and close switch b. Hence, the voltage is stepped up from E to the total voltage value E from which the attenuation of the voltage is resumed as time passes. The voltage variations in amplitier 7 and relay 10 respectively are shown in FIG. 6 and FIG. 7. The output signals are transmitted to an amplifier 11, and, finally, a comparison circuit 13 via a rectifier filter circuit 12, being subjected to amplification, rectification into direct current, and conversion into pulses P and P The pulses P and P are produced when the voltage is stepped up from E to E and reduced from E to E respectively, as shown in FIG. 9. A square-wave generator circuit 14-, for gating the signals to the relay 16, receives the pulses P and P and transmits resulting square-wave gate signals to a counter 17 through a Q-f selector 15. In this counter the number of the pulses is counted for the time interval from P to P the level displacement of which is shown in FIG. ll, in which the amplitude is prevented from exceeding the predetermined value by a limiter (not shown). On the contrary, the gate signal causes relay 10 to open switch B and close switch A. The Q-f selector 15 is used to switch the counter 17 when the damping factor and the resonant frequency are examined. In the preferred example, either Q or f is selected as desired, before the specimen is struck. The value Q is determined by multiplying the number indicated on the counter 17 by the reciprocal of the voltage-division from the divider 9. The value i is determined by the number indicated on the counter 17, which is operated by a different gate signal from a time-base generator 16.

The voltage value to be predetermined by the divider 9 is preferably selected in accordance with the estimated magnitude of the damping factor to be measured. Advantageously, the comparison circuit 13 comprises a multivibrator having two transistors or vacuum tubes, so connected that positive feedback is possible, and the squarewave generator circuit 14 comprises a univibrator in which either state of operation is quickly exchangeable to the other.

Among advantageous features of the invention are the following:

The mechanical and electrical construction of the unit employed can be greatly simplified, using manual operation to tap the surface of the specimen. The specimen is supported with stability, resting on its fulcrum, as described above; this is quite ditferent from the prior art method. Thus, a very heavy specimen, such as an axle for train wheels, can be examined without difiiculty. A specimen is not limited in weight, shape and dimension, thus providing no limitation to the applications. The resulting data is very accurate, because the damping factor is measured by counting the number of oscillations occuring during the differential level in the process of free-decay vibration, regardless of the actual magnitude of the damping factor. The damping factor and the resonant frequency are measured simply through the counting of the number of vibrations during the selected time intervals, the resulted values of which are given on a counter through a step of a mathematical operation, thus rendering the use of the invention simple and inexpensive.

I claim:

1. A method of non-destructive examination of the material and the structural condition of a specimen by measuring its damping factor, comprising:

(a) supporting the specimen in such a manner as to be capable of free-decay vibrations,

(b) striking the specimen to cause vibration,

(c) converting the vibrations thus caused to an equivalent electrical signal,

(d) dividing said equivalent electrical signal to provide a series of simultaneously occurring signals of an amplitude reduced from but proportional to that of said equivalent electrical signal and of the same frequency as said equivalent electrical signal, the first of said series of signals being smaller in amplitude than said equivalent electrical signal, each succeeding one of said series of signals being smaller than its immediately preceding signal,

(e) selecting a second signal from said series of signals,

said second signal being selected so that the relative difierence in amplitude between said second signal and said equivalent electrical signal is in approximate accordance with the estimated magnitude of the damping factor to be measured, and

(f) counting the number of vibrations which occur from the time instant when said second signal attenuates to a predetermined value until the time instant when said equivalent electrical signal attenuates to the same predetermined value, to thereby measure the damping factor of the specimen.

2. The method claimed in claim 1 additionally comprising the further step of counting the number of vibrations occurring in a predetermined time to thereby measure the resonant frequency of the specimen.

3. A method according to claim 1 wherein the step of supporting the specimen comprises:

(a) supporting a pair of filaments substantially parallel with tension on opposite ends of the filaments, and

(b) placing the specimen on the filaments in such a manner that it is supported by the filaments at its characteristic nodes.

4. A method according to claim 1 wherein the step of converting the Vibrations comprises:

(a) assuring the said specimen is provided with a magnetizable section,

(b) positioning an electromagnetic pickup adjacent to said magnetizable section, and

() using an electrical signal from said pickup as said equivalent electrical signal.

5. A method according to claim 1 wherein the step of counting the number of vibrations comprises:

(a) initially switching to receive said equivalent electrical signal,

(b) rectifying and filtering said equivalent electrical signal to obtain one DC. signal proportional to the R.M.S. value of said equivalent electrical signal,

(c) comparing said R.M.S. value of said one signal with a predetermined signal level,

(d) subsequently, when said one DC. signal reaches said predetermined signal level, switching to stop receiving said equivalent electrical signal and to start receiving said signal,

(e) beginning to count said vibrations when beginning to receive said second signal,

(f) rectifying and filtering said second signal to obtain another DC. signal proportional to the R.M.S. value of said second signal,

(g) comparing the R.M.S. value of said other signal with said predetermined signal level, and

(h) ceasing to count said vibrations when said other signal reaches said predetermined level.

References Cited UNITED STATES PATENTS 3,513,690 5/1970 Pellerin et a1. 73-67 RICHARD C. QUEISSER, Primary Examiner A. KORKOSZ, Assistant Examiner 

